Modulated oscillator



Fcb. 20, 1934. p SCQFIELD 1,947,805

MODULATED OSC ILLATOR Filed May 31, 1932 INVENTOR, PHIL/P F. SC OF/ELD.

A TTORNE Y.

Patented Feb. 20, 1934' UNITED STATES 1,947,805 MODULATED' OSCILLATOR Philip 1i Scofield, Palo Alto, Califi, assignor to Heintz & Kaufman, Ltd., San Francisco, Calif., a corporation of Nevada Application May 31, 1932. Serial No. 614,479 reclaims. (or. 179-171) My invention relates to modulated oscillators, and particularly to the modulation of oscillating systems comprising a pair of cooperating tubes, such as oscillators of the Simpson type, as described in Patent No. 1,775,327 and various pushpull oscillators.

Among the objects of my invention are: To provide a modulated oscillator giving a high percentage of modulation; to provide a modulated oscillator requiring relatively small modulating power; to provide an oscillator whose stability is not impaired by modulation; to provide a modulating system wherein the circuits are of extreme simplicity; and to provide a modulaing system which is light in weight and therefore adapted to aircraft use.

Other objects of my invention will be apparent or will be specifically pointed out in the description forming a part of this specification, but I do not limit myself to the embodiment of my invention herein described, as various forms may be adopted within the scope of the claims.

The single figure of the drawing is a circuit diagram showing the modulating system of my invention as applied to an oscillating circuit of the Simpson type.

Two general types of circuit are used in modulating radio frequency oscillation. The first type, or grid modulated" oscillator, operates by varying the potential of the control electrode of a vacuum tube at the modulating frequency, simultaneously with its radio frequency potential variation. The second type, or plate modulated oscillator, operates by varying the plate current at the modulating frequency. 7

The first type has the advantage of requiring low modulating frequency power, which permits light equipment, but in the past this has been offset by the fact that the percentage of modulation obtainable has been relatively low, the modulation has interfered with the stability of the oscillator, and there has frequently been relatively large distortion in the modulated output of the tube. Plate modulation, on the other hand, has been capable of giving high percentages of modulation and low distortion, but has required that the modulating equipment have an excess of capacity over the oscillating equipment, and the apparatus as a wholehas been correspondingly bulky, heavy, and costly.

Oscillators may also be divided into two classes in accordance with the method of grid bias employed. The first type comprises those wherein a resistor or grid leak connects the control electrode with the cathode, and the mean potential of the grid with respect to the cathode is determined by the voltage drop in the resistorp'roduced by current from cathode to control elec trode.

Since the amount of the grid current is determined by the extent to which the grid swings positive in oscillation, and hence by the amplitude of the oscillation, this method permits the control electrode to assume it's own best operating potential, with corresponding stability and reliability of operation. This method of bias is, therefore, the one which is probably in most general use, particularly in oscillators operating at the higher frequencies. The second. method provides a definite predetermined bias to'the control electrode from a battery or other source, applied to the grid through a circuit of relatively low resistance. This method is in less general use than the self-biasing method because it is more difficult to start oscillation in the system when this method is used, and because more careful adjustment is necessary with this system in order to provide the proper bias. The method is, however, generally called into service when grid modulation of the oscillator is used. I v

In general terms, the modulating system of my invention comprises an oscillator having a pair of vacuum tubes each of which is excited by the other in so far as the oscillating potential on the control electrode is concerned. One of these tubes is provided with a substantially constant bias, preferably through a resistor. The second tube is provided with means for applying a modulating frequency potential to the grid,

superposed upon the oscillating potential supplied from the other tube. Such bias as is required by the control electrode of this second tube is provided by a battery or other equivalent means, the circuitbeing so arranged that the high frequency is not grounded through the modulating potential source. When-the proper operating potentials are applied to the electrode of the oscillator, oscillation starts smoothly and certainly due to the action of the resistor bias on the first tube. '.The modulating potential applied to the control electrode of the other tube, however, controls the intensity of oscillation, but owing to the 'fact' that the first tube is self-biased, this control is exercised without loss of stability in the performance of the appara tus. Plate current'to both tubes is preferably supplied through a radio frequency choke coil, so that sudden changes in total current cannot occur. This limits the peak current drawn b'y either tube to the sum of the average currents of both, and hence the radio frequency output of both tubes is controlled by the modulating potential.

The drawing illustratesmy invention as ap-' plied to an oscillator of the Simpson type. The oscillator comprises a pair of triodes 1, 2, having. cathodes 4 and 5 respectivelyfheated by current supplied through a common circuit 6' from a battery or othersuitable source 7. The :plate v 9 of tube 'l'isconnected to the plate 10 of'tub'2' through an oscillating circuit comprising similar inductance coils 11 and 12 in series, the coils being bridged or shunted by a variable condenser 14, by means of which the circuit is tuned. The two coils are preferably arranged for zero mutual inductance, this being indicated in the figure by the dotted lines 15, representing shields surrounding the individual coils. r

The control electrode 16 of tube 1 is connected through a blocking condenser 17 and leads 19 and 20 with the plate or anode 10 of tube 2. The control electrode 21 of tube 2 is, in like manner, connected with the plate of tube 1 through a blocking condenser 22, and leads 24 and 25.

Plate current is supplied to both tubes from a suitable source 26, through a choke coil 27. This choke coil has as high an impedance as is feasible, preferably in excess of that of the tuned circuit 11, 12, 14, so that its point of attachment to the oscillating system does not materially affect the distribution of potentials thereon. Therefore, although this coil is shown as connected at the' junction of the two inductors 11 and 12, the circuit will operate equally well if the connection be made at the plate of either tube.

Power may be withdrawn from the oscillating circuit through a coil 29 coupled with coil 11, or by any other desired coupling means, and supplied to an antenna, amplifier, or other load.

Control electrode 16 of tube 1 is biased through the high resistor or grid leak 30. The biasing circuit for the second tube comprises a battery 31 or other source of biasing potential, connected in series with the secondary coil 32 of a modulating transformer and a radio frequency choke coil 34, Whose high potential end connects to the control electrode 21. The primary coil 35 of the modulating transformer is shown as supplied by the microphone 36 connected in series with the microphone battery 37, but it isto be understood that this circuit may be replaced by any suitable source of modulating frequency potential, such as a condenser microphone and audio frequency amplifier, or the like.

The operation of the circuitis as follows:

When plate potential is first applied to the tube through the radio frequency choke 27, the control electrode 21 is negative due to the potential applied from the source 31, while the control electrode 16 is at zero potential or slightly positive, owing to its connection with the cathode circuit through the resistor 30. The tube 1 therefore draws a much heavier space current than does tube 2, causing a fall of potential on the plate 9. This decrease in potential is transferred through the connection 24 and blocking condenser 22 to control electrode 21, forcing this control electrode still farther negative, the effect building up until tube 1 is drawing maximum current and tube 2 reaches cut-oil, or practically zero current, at which time control electrode 21 starts to reassume its less negative, normal or bias potential, since it is no longer being swung by increasing current in tube 1. The current through tube 2 therefore starts to increase, thereby reducing the potential of the plate 10 and the grid 16, so that the action of the two tubes is now reversed, tube 2 carrying maximum current and tube 1 minimum current. This cycle of operation is then repeated indefinitely, the plate of each tube exciting the control electrode of the other to produce oscillations whose period is operating curve of the tube.

determined by the natural period of the oscillating circuit comprising the-coils 11 and 12 and the condenser 14.

It should be noted that the choke 27 prevents any material sudden or high frequency change in the total current supplied to the two tubes; although slow or audio frequency changes in total current are free to occur. Hence, any sudden increase in space current flowing in one tube is accompanied by a corresponding decrease in space current flowing in the other tube. This means that the peak current which flows in either tube is limited to the sum of its average current and the difference between average and minimum current in the other tube.

The minimum current in both tubes is substantially zero. The average current for each tube depends both on its bias and the amplitude of the radio frequency swing. Changes in bias on tube 2, by limiting the amplitude of the radio frequency swing necessary to reach outoff or minimum current, also change the average current and hence the maximum current flowing in tube 1.

Modulating frequency potentials applied to control electrode 21 of tube 2 through the transformer secondary 32, since they are long in period in comparison with the radio frequency oscillation, are effectively changes in bias or mean potential, and serve thus to modulate both tubes.

In order that this may occur it is not necessary that the biasing potential applied from battery 31, which may be referred to as the static bias, be at any particular point on the The system will operate whether the static bias be on the socalled straight line portion of the tube curve, at the cut-off portion, or even below cut-off, although the circuit operates more satisfactorily when tube 2 is biased to cut-01f or some- What above. Extremely high percentages of modulation may be obtained with this system. Instability does not develop nor does objectionable distortion occur even with modulation percentages over 80.

The method of modulation can be used in an ordinary push-pull oscillating circuit, wherein a single coil is substituted for the uncoupled coils 11 and 12, or where the choke 2'7 is omitted. In the latter circumstance, however, the only advantages gained are stability of oscillation and freedom from distortion, since the modulation of tube 1 by limitation of maximum current due to the cut-off of tube 2 does not occur to the same extent, and the high degree of modulation obtainable with thepreferred form of circuit does not obtain.

It is to be understood that in referring to the "grid? of a tube, the term is used merely for convenience, and any electrode exercising a control over the space current of the tube is included, whatever its structural form.

I claim:

1. A modulated oscillator comprising a pair of vacuum tubes each having a control electrode, a plate circuit for each of said tubes, a connection from the plate of each of said tubes to the control electrode of the other tube, means for varying the potential of the control electrode of one of said tubes at a modulating frequency, and means for maintaining a substantially constant bias on the control electrode of the other tube.

2. A modulated oscillator comprising a pair of vacuum tubes each having a control electrode, means connected to each of said tubes for exciting the control electrode of the other tube to produce oscillation, means for applying a modulating frequency potential to the control electrode of one of said tubes, and means for applying a substantially constant bias to the control electrode of the other tube.

3. A modulated oscillator comprising a pair of vacuum tubes each having a control electrode, means connected to each of said tubes for exciting the control electrode of the other tube to produce oscillation, means for applying a modulating frequency potential to the control electrode of one of said tubes, and a resistor connected to bias the control electrode of the other tube.

4. A modulated oscillator comprising a pair of tubes each having a control electrode and an anode, a resonant circuit connecting the anodes of said tubes, a connection including a blocking condenser from the control electrode of each of said tubes to the anode of the other tube, means for applying a modulating frequency potential to the control electrode of one of said tubes, and a resistor connected to bias the control electrode of the other tube.

5. A modulated oscillator comprising a pair of tubes each having a control electrode and an anode, a resonant circuit connecting the anodes of said tubes, a connection including a blocking condenser from the control electrode of each of said tubes to the anode of the other tube, a circuit including a source of modulating potential in series with an impedance element connected to the control electrode of one of said tubes to apply modulating frequency potential thereto, the impedance of said impedance element being of at least as high an order of magnitude as that of said resonant circuit to the natural frequency of said circuit, and a resistor connected to bias the control electrode of the other tube.

6. A modulated oscillator comprising a pair of tubes each having a control electrode and. an anode, a resonant circuit connecting the anodes of said tubes, a connection including a blocking condenser from the control electrode of each of said tubes to the anode of the other tube, a circuit including a source of modulating frequency potential in series with a radio frequency choke coil connected to apply modulating frequency potential to the control electrode of one of said tubes, and a resistor connected to bias the control electrode of the other tube.

7. A modulated oscillator comprising a pair of vacuum tubes each having a cathode, an anode and a control electrode, a resonant circuit connecting said anodes, a connection including a blocking condenser from the control electrode of each of said tubes to the anode of the other tube, a circuit including a source of modulating frequency potential connecting the cathode of one of said tubes with the control electrode thereof, and a biasing resistor connecting the cathode of the other tube with its control electrode.

8. A modulated oscillator comprising a pair of vacuum tubes each having a cathode, an anode, and a control electrode, a resonant circuit connecting said anodes, a connection including a blocking condenser from the control electrode of each of said tubes to the anode of the other tube. a circuit including a source of modulating frequency potential in series with a radio frequency choke coil connecting the cathode of one of said tubes with the control electrode thereof, and a biasing resistor connecting the cathode of the other tube with its control electrode.

9. A modulated oscillator comprising a pair of vacuum tubes each having a cathode, an anode, and a control electrode, a resonant circuit connecting said anodes, a connection including a blocking condenser from the control electrode'of each of said tubes to the anode of the other tube, a circuit including a source of modulating frequency potential in series with a radio frequency choke coil and a source of biasing potential connecting the cathode of one of said tubes with the control electrode thereof, and a biasing resistor connecting the cathode of the other tube with its control electrode.

10. A modulated oscillator comprising a pair of vacuum tubes each having a cathode, an anode, and a control electrode, a resonant circuit comprising a pair of coils having substantially zero mutual inductance connected in series and shunted by a condenser connecting said anodes, means for supplying space current 'to said tubes, said means including a radio frequency choke coil to prevent sudden changes in the total space current in said tubes, a connection including a blocking condenser from the control electrode of each of said tubes to the anode of the other tube, a biasing resistor connecting the control electrode of one of said tubes with the cathode thereof, and means for applying a potential of modulating frequency to the control electrode of the other tube.

11. A modulated oscillator comprising a pair of vacuum tubes each having a cathode, an anode, and a control electrode, a resonant circuit comprising a pair of coils having substantially zero mutual inductance connected in series and shunted by a condenser connecting said anodes, means for supplying space current to said tubes, said means including a radio frequency choke coil to prevent sudden changes in the total space current in said tubes, a connection including a blocking condenser from the control electrode of each of said tubes to the anode of the other tube, a biasing resistor connecting the control electrode of one of said tubes with the cathode thereof, and a source of modulating frequency potential connected in series with a radio frequency choke coil between 1tshe control electrode and cathode of the other ube.

12. A modulated oscillator comprising a pair of tubes connected to oscillate in opposite phase relationship, means for supplying space current to said tubes, means for preventing radio frequency changes in the total space current supplied to said tubes, and means for varying, at a modulating frequency, the limit of radio fre quency swing in space current of only a single one of said tubes.

13. A modulated oscillator comprising a pair of tubes connected to oscillate in opposite phase relationship, means for supplying space current to said tubes, means for varying at a modulating frequency, the radio frequency changes in space current in only a single one of said tubes, and means for limiting the radio frequency increases in space current in each tube to current withdrawn from the other tube.

PHILIP F. SCOFIELD. 

